[R] non-linear model parameterization

[Irene Mantzouni]

Dear all,
 
I would like to fit a non-linear model of the form:
y=g*x/(a+b*x)
with nls().
However this model is somehow overparameterized and I get the error message about
singular gradient matrix at initial parameter estimates.
What I am interested in is to make inference about parameters b and g, so this has to be taken into account in the model formulation. 
What options do I have?
Also, how is it possible to fit a partially linear model? 
 
Thank you!!
 
Irene Mantzouni
----------------
PhD student
DIFRES